1. Field of the Invention
The present invention relates to control and safety devices for instantaneous gas appliances such as, for example, water heaters, and is more particularly concerned with an improved small safety pressure cap used to control the flow of gas to the pilot of a gas appliance.
2. Description of the Priot Art
Flame-detection devices whose function is to react to the extinguishing of a pilot and stop the admission of gas to the main burner of an appliance, such as an instantaneous water heater or bath heater, operate in accordance with several known and widely applied principles. Among these prior art devices there is a system that not only ensures this safety function, but also permits it to function as a device which directly controls a valve. Such a system includes a pressure responsive cap consisting in known fashion of a small watertight chamfer or bulb filled with air and placed in the flame which is to be detected, and a capillary tube connected to a nipple closed by a small expansible membrane which can be inflated under the effect of the expansion of the air contained in the bulb and which can operate a valve for the admission of gas into the burner.
This system has the advantage of being simple. However, because of the mechanical energy which would be required if it were utilized as a main control device for the admission of gas to the appliance, it is preferable to use such a device downstream of the main control valve as a gas-monitoring and regulating unit, particularly in appliances which produce instantaneous hot water and which operate without a permanent pilot.
In such devices, after a certain period of operation, a drawback arises which is characterized by the fact that, due in particular to the porosity of the small expansible membrane and the difficulty in obtaining effective seals between the bulb and the capillary tube due to the hard rollers and the air contained in the pressure responsive cap tends to escape slowly. As a result, the expansion of the air contained in the pressure responsive cap is no longer sufficient to inflate the membrane and, thereby, to operate the valve that admits gas to the burner.